Backscatter A [8101]--Offshore Bolinas, California

This part of DS 781 presents data for the acoustic-backscatter map of the Offshore of Bolinas map area, California. Backscatter data are provided as separate grids depending on mapping system or processing method. The raster data files is included in "BackscatterA_8101_2004_OffshoreBolinas.zip", which are accessible from http://pubs.usgs.gov/ds/781/OffshoreBolinas/data_catalog_OffshoreBolinas.html. The acoustic-backscatter map of the Offshore of Bolinas map area, California, was generated from backscatter collected by California State University, Monterey Bay (CSUMB), by Fugro Pelagos, and by Moss Landing Marine Laboratory (MLML). Mapping was completed between 2004 and 2010, using a combination of 200-kHz and 400-kHz Reson 7125, and 244-kHz Reson 8101 multibeam echosounders, as well as 468-kHz SEA SWATHPlus and 250-kHz GeoSwath interferometric systems. Moss Landing Marine Laboratory mapped the nearshore region north of Bolinas in 2004 prior to the California Seafloor Mapping Program (CSMP). The nearshore region from south of Bolinas Lagoon to Stinson Beach was mapped by Fugro Pelagos in 2009 for a project outside of the CSMP and only bathymetry data were collected. Therefore, note that the shaded relief map coverage (see Bathymetry Hillshade--Offshore of Bolinas, California, DS 781) does not match the acoustic-backscatter map coverage (see Backscatter A-E--Offshore of Bolinas, California, DS 781). Within the acoustic-backscatter imagery, brighter tones indicate higher backscatter intensity, and darker tones indicate lower backscatter intensity. The intensity represents a complex interaction between the acoustic pulse and the seafloor, as well as characteristics within the shallow subsurface, providing a general indication of seafloor texture and sediment type. Backscatter intensity depends on the acoustic source level; the frequency used to image the seafloor; the grazing angle; the composition and character of the seafloor, including grain size, water content, bulk density, and seafloor roughness; and some biological cover. Harder and rougher bottom types such as rocky outcrops or coarse sediment typically return stronger intensities (high backscatter, lighter tones), whereas softer bottom types such as fine sediment return weaker intensities (low backscatter, darker tones). These data are not intended for navigational purposes.本数据集DS 781的本章节提供了加利福尼亚州博利纳斯近海测绘区的声学反向散射（acoustic-backscatter）地图数据。反向散射数据根据测绘系统或处理方法的不同，以独立网格文件的形式提供。栅格数据文件包含在"BackscatterA_8101_2004_OffshoreBolinas.zip"压缩包中，可通过http://pubs.usgs.gov/ds/781/OffshoreBolinas/data_catalog_OffshoreBolinas.html获取。 加利福尼亚州博利纳斯近海测绘区的声学反向散射地图，由加州州立大学蒙特利湾分校（California State University, Monterey Bay, CSUMB）、Fugro Pelagos公司以及莫斯兰丁海洋实验室（Moss Landing Marine Laboratory, MLML）采集的反向散射数据生成。测绘工作于2004年至2010年间完成，使用了200kHz与400kHz的Reson 7125、244kHz的Reson 8101多波束测深仪（multibeam echosounders），以及468kHz的SEA SWATHPlus和250kHz的GeoSwath干涉测量系统。2004年，莫斯兰丁海洋实验室在加州海底测绘计划（California Seafloor Mapping Program, CSMP）启动前，完成了博利纳斯以北近岸区域的测绘。2009年，Fugro Pelagos公司为一项非加州海底测绘计划的项目，完成了博利纳斯泻湖以南至斯廷森海滩的近岸区域测绘，此次仅采集了水深数据。因此需注意：阴影晕渲地图覆盖范围（详见《加利福尼亚州博利纳斯近海地形晕渲图，DS 781》）与声学反向散射地图覆盖范围（详见《加利福尼亚州博利纳斯近海反向散射A-E图，DS 781》）并不一致。在声学反向散射影像中，亮度越高代表反向散射强度越高，亮度越低则代表反向散射强度越低。该强度反映了声脉冲与海底及其浅地下层特性之间的复杂相互作用，可用于大致指示海底纹理与沉积物类型。 反向散射强度取决于声源级、海底成像所用的声波频率、掠射角、海底的组成与特性（包括粒径、含水量、体积密度及海底粗糙度），以及部分生物覆盖物。质地更坚硬、表面更粗糙的海底类型（如岩石露头或粗颗粒沉积物）通常会返回更强的信号（高反向散射，对应较亮色调），而质地较软的海底类型（如细颗粒沉积物）则会返回较弱的信号（低反向散射，对应较暗色调）。本数据集不适用于航海导航用途。